High temperature resistance thermometer



Sept. 10, 1963 L. J. GEE 3,103,641

HIGH TEMPERATURE RESISTANCE THERMOMETER Filed March 20, 1961 INVENTOR.'- Z520) 61. .4

I WMM 3,103,641 j w HIGH TEMPERATURE RESISTANCE THERMOMETER Leroy J. Gee, Palo Alto, Calif., assignor to American Radiator & Standard Sanitary Corporation, New York, N.Y., a corporation of Delaware Filed Mar. 20, 1961, Ser. No. 96,793

i 2 Claims. (Cl. 338-28) This invention relates to and in general has for its object. the provision of a high temperature. resistance thermometer, and more specifically to a probe for such a 1 United States Patent Office thermometer which will operate within a range of 3500- I 4000 F. 2 v v The need for adequate means of measuring high temperatures in an oxidizing atmosphere has existed for some time. For some applications such special methods as pyrometric devices and pneumatic probes have been utilized. However, temperature sensors having the necessary characteristics (i,e., accuracy, stability, long life, repeatability, and ease of application), although commonly available for the lower and middle-temperature regions, are presently not available for'the high-temperature regions. Considerable effort has been devoted to this particular problem, especially in the area of thermoelectric methods. At least seven thermocouple combinations presently having calibrations up to 4000" F. are described in the literature. Unfortunately, however, none of these can be used inoxidizing environments. Thus, the combination of high temperatures and oxidizing atmospheres presents an obstacle that has not yet been satisfactorily overcome. Because of this lack of available thermoelectric materialsthat can be directlyused in hightemperat-ure oxidizing atmospheres, much effort has been expanded in developing sheaths and coatings to protect thermocouple junctions from the effects t oxidation. These efforts have not been completely successful because of the 'difiiculty of obtaining completely gasatight and impermeable sheaths. Where these approaches are successful, the performance characteristics of the thermocouple are usually compromised because of the necessary addition of considerable mass and-size to the temperaturesensitive areas. I 1

In addition to the requirement thata temperature probe operate within such a high temperature range, certain segments of the industryhave specified that such a probe have the following characteristics:

(1) 100 hours of life at 4000 F.

1000 hours of life at 3500" F.

i1% accuracy for the first 100 hours :2% accuracy for the following 1000 hours Operating environment:

(a) Quiescent air (b) 1 x 10 n/cm. -sec. neutron flux Geometry:

30-in. length 0.063 in. OD.

(5) Thermal shock'must withstand temperature change of 1000 'F./sec. g I

(6) Time response-2 to 3 seconds in quiescent air (7) Signal outputat least 20 mv. at 3500 F.

Various known methods'of' temperature sensing are: expansion, pressure, and resistance thermometers, thermocouples; pyrometers. Expansionand pressure-type thermometers are generally applicable to the lower temperature regions where the physical characteristics of the sensor material, such as linear expansion, volumetric expansion, or vapor pressures, are accurately known. At extremely high temperatures, the uncertainty of these physical properties and the difficulty of finding suit-able awash material to serve either as the sensor or to contain the sensor usually eliminate these particular methods for consideration. At the other extreme, pyrometric devicesare particularly applicable to high-temperature measurements but require that the source emissivity be known or that block-body conditions exist before accurate measurements can be obtained. In the present applicaiton, the geometrical limitation precludes pyrometric means.

concept. In thepast decade, a major portion of the research Work in the field of high-temperature sensing couples, now extensively utilized.

More specifically, the object of this invention-ais the provision of a probe for a temperature resistance ther mometer which can be made to substantially meet the specifications above set forth.

A further object of this invention is the provision of a probe of the character above described, madeup of a I pair of spaced electrical leads bridged by a refractory metal oxide temperature sensor and wherein the leadsv The invention possesses other advantageous features,

some of which, with the foregoing, will be set forth at length in the following description Where that form of the invention which has been selected for illustration, in the drawings accompanying and forming a parto'fthe present specification is outlined in full. In said drawings,

one form of the invention is shown, but it is to be understood that it is not limited to such form, since'the invention as set forth in the claims may be embodied in other forms. a 1

Referring to the drawings: a 1 FIG. 1 is a longitudinalmidsection of a high temperature resistance thermometer probe embodying the objects of my-invention.

FIG. 2 is a vertical section taken on the section line 2.2-ofFIG. 1. j

The probe illustrated in these two figures consists of a pair of spaced, parallel electrical leads 1 and 2 conveniently made of a refractory metal such as iridium, and of semicircular cross-section. Bridging the left-hand pain i of the ends of the leads 1 and 2 and electrically bonded thereto is a domed temperature sensor or tip 3 made of I Zir-conia or other refractory metal oxide having a negative temperature coefiicient of resistivity. Surrounding each of the leads 1 and 2 is a first inner film 4 of a getter material such as rhenium or tantalum and a second outer film or coating 5 of a refractory metal oxide such as thoria or beryllia.

in producing a structure of the configuration shown in FIGS. 1 and 2, iridium wire from which the leads 1 and 2 i are to be made, and having an outer diameter in the order of 0.02 to 0.03 inch, are ground on one sideso as to g produce a wire of semi-circular cross-section such as illustrated in FIG. 2. A first and inner film of tantalum or rhenium is flame-sprayed on the leads, and then a second and outer film of beryllia or thoria in the order of 0.005 inch thick is sprayed over the getter film. Following this, the two flat surfaces of the coated leads are placed together and this assembly is flame-sprayed with a supplemental .fil m of the same oxide previously applied to the leads. This latter and supplemental film or coat- Patented Sept. 10,1963

It an" pears then that the solution lies in either some unique thermoelectric approach or the resistance-thermometer has been devoted to the development of suitable thenmov ing' serves to bind the two leads together and also serves Finally, one pair of the ends of the coated and bonded leads are flamef sprayedwith z irconia so as to bridge the lead ends and form the temperature-sensing element of theprobe. Al-

as a protective coatingand insulator.

though .this method can be conveniently used for producinga probe of the character above described, other methods can of course be resorted to.

H 'The refractory metal oxides above referred to have melting points "and limits of usefulness in an oxidizing atmosphere indicated in the following table V Limit of Melting Point Useiulness'in (approximate) oxidizing (Degrees F.) Atmosphere (Degrees F.)

All of these oxides are stable ati liigh temperatures in oxidizing atmospheres, and allot them possess a negative temperature coeffi-cient of resistance. Here it should 1 be noted that zirconia becomes a conductor at about 3600" 'F.,fa known fact that limits its usefulness as athermocoupleprotective material. However, ,itis this very characteristiethat makes it particularly suitable as a temperatore-sensitive; element in a resistance type of thermometer such as herein described. 7 1

v In fthe probe 'ofmy invention the iridium electrical le-ads'l and Zhave a'much lower electrical resistance than the zirconia temperature sensor or tip 3, the latter serves as the primary resistance and its value will of course be determined by the temperature being sensed.

Furthermoresthe dimensions of the sensor 3 can be ad- 'justed to some extent in order to achieve the best possiblecombination of resistance values and time response. 1 Care'should be taken to insure a good electrical junction between the zirconia sensor? andthe leads 1 and 2,

"for any'tendency to separategwill introduce resistance into (the measuringcircuit associated with the probe and which 1 will introduce erroneous results.

7 leads 1 and 2 and also have a strong tendency to iurther seal the outer oxide insulating film or coating.

As a result of this construction then it is possible to meet the specifications above referred to with respect to operating temperatures andenvironment, life, accuracy, time response, and signal output.

I claim:

1'.'In a high-temperature resistance thermometer; a'

pair of'spaced refractory metal electrical leads; a temperature sensor bridging and bonded to said electrical leads, said sensor being rnade of a refractory metal oxide selected from the group consisting of beryllia, magnesia,

thoria, and zinconia and having a negative temperature coefiicient' of resistivity; the electrical resistance of said leads being lower than they electrical resistance of said sensor; and said leads being coated with a first inner film of a getter material-selected from the group consisting of rhenium and tantalum, and said leads being coated with a second outer film of a refractory oxide.

, 2. Ina high-temperatureresistance thermometer: apair I of closely spacedelongated, refractory metal electrical leads each of semi-circular cross secti'on. with the flat sides of said leads being adjacent each other, and with one end of one lead'terminating adjacent one end of the other lead; a domed temperature sensor bridging and bonded to saidadjacent ends of said electrical leads, said sensor being made of a refractory metal oxide having a negative temperaturecoeflicient of resistivity, the electrical resistance of said leads being lower than the elec-,

I By coating the iridium leads 1 and 2 with an, under coating orfilm of; a getter material and then with an outer dense film of thoria or beryllia, the leads are protected against" oxidation. Anyoxidizing elements which may eventually penetrate the thoriaor beryllia coating will attack the getter film. Rhenium and tantalum are particularly. suitable for this purpose; for they form volatile oxides, serving as a neutral barrier around the iridium trical resistance of said sensor, each of'sai'd leads being coated completely therearound with a first inner film of getter material bonded thereto and extending from said sensor toa point substantially remote from said sensor,

said leads beingcoated completely therearound and therebetween with a second outer film of a refractory oxide extending the length of said firstinner 9 References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Yarwood: High Vacuum Technique, 1st ed, 1943, pp. 38-49. 1 

1. IN A HIGH-TEMPERATURE RESISTANCE THERMOMETER: A PAIR OF SPACE REFRACTORY METAL ELECTRICAL LEADS; A TEMPERTURE SENSOR BRIDGING AND BONDED TO SAID ELECTRICAL LEADS, SAID SENSOR BEING MADE OF A REFRACTORY METAL OXIDE SELECTED FROM THE GROUP CONSISTING OF BERYLLIA MAGNESIA THORIA, AND ZIRCONIA AND HAVING A NEGATIVE TEMPERATURE COEFFICIENT OF RESISTIVITY; THE ELECTRICAL RESISTANCE OF SAID LEADS BEING LOWER THAN THE ELECTRICAL RESISTANCE TO SAID SENSOR; AND SAID LEADS BEING COATED WITH A FIRST INNER FILM OF A GETTER MATERIAL SELECTED FROM THE GROUP CONSISITNG OF 